Solution having biocidal activity

ABSTRACT

A method and apparatus for the electrochemical treatment of an aqueous solution in an electrolytic cell is described. Output solution having a predetermined level of available free chlorine is produced by applying a substantially constant current across the cell between an anode and a cathode while passing a substantially constant throughput of chloride ions through the cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/663,079, filed on Sep. 16, 2003, which is a divisional of U.S.application Ser. No. 09/633,665, filed Aug. 7, 2000, which claimspriority to GB Application No. 9918458.2, filed on Aug. 6, 1999 and GBApplication No. 9927808.7, filed on Nov. 24, 1999, all of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates, among other aspects, to a method ofoperating an electrochemical cell to produce a biocidal solution andapparatus for producing a biocidal solution by way of the electrolytictreatment of an aqueous chloride solution.

In hospitals it is important to provide appropriate levels of sterility,particularly in operating theatres and other situations where invasivetreatments are performed. Surgical instruments and other apparatus mustbe sterilised or disinfected, depending on their application, before usein order to reduce the risk of bacterial infection. One method ofsterilisation is the application of heat and pressure in an autoclave.However, this is not suitable for some medical apparatus, such asheat-sensitive endoscopes.

A typical method employed for reprocessing heat sensitive instrumentsinvolves the use of chemical biocides, such as glutaraldehyde. This canbe unsatisfactory due to improper or incomplete disinfection.Furthermore, exposure to glutaraldehyde fumes can cause asthma anddermatitis in healthcare staff. Also, glutaraldehyde is believed to haverelatively low sporicidal activity. Moreover, other disinfectants, suchas chlorine dioxide and peracetic acid may suffer from similar handlingproblems as glutaraldehyde.

For some years, it has been known that electrochemical activation ofbrine produces a super-oxidised water which is suitable for manyapplications including general disinfection in medical and veterinaryapplications and the sterilisation of heat-sensitive endoscopes. Therehas been a recent interest in the use of super-oxidised water as adisinfectant because of its rapid and highly biocidal activity against awide range of bacteria, fungi, viruses and spores. Also, super-oxidisedwater is an extremely effective sterilising cold non-toxic solutionwhich is free from highly toxic chemicals, thereby presenting reducedhandling risk.

GB 2253860 describes the electrochemical treatment of water through anelectrolytic cell. Co-axially arranged cylindrical and rod electrodesprovide anode and cathode (working and auxiliary) flow chambers whichare separated by a porous membrane made of a ceramic based on zirconiumoxide.

Water is fed from the bottom to the top of the device through theworking chamber. Simultaneously, water having a higher mineral contentflows through the auxiliary chamber to a gas-separating chamber. Anelectric current is passed between the cathode and anode through thewater in both chambers and the porous membrane separating the chambers.Water flowing through the auxiliary chamber recirculates to theauxiliary chamber by convection and by the shearing forces applied tothe water through the rise of bubbles of gas which are generated on theelectrode in the auxiliary chamber. The pressure in the working chamberis higher than that in the auxiliary chamber, and gaseous electrolysisproducts are vented from the gas-separating chamber by way of agas-relief valve. A change of working mode from cathodic to anodic watertreatment is achieved by changing polarity.

This electrolytic process acts on salts and minerals dissolved in thewater, such as metal chlorides, sulphates, carbonates andhydrocarbonates. Where the working chamber includes the cathode, thealkalinity of the water may be increased through the generation ofhighly soluble metal hydroxides. Alternatively, the electrolytic cellmay be switched so that the working chamber includes the anode, in whichcase the acidity of the water is increased through the generation of anumber of stable and unstable acids.

A similar electrolytic cell is described in GB 2274113. This cellincludes two coaxial electrodes, separated by an ultra-filtrationdiaphragm (porous membrane) based on zirconium oxide, thereby defining apair of coaxial chambers. A current source is connected to theelectrodes of a plurality of cells via a switching unit to enablepolarity alteration of the electrodes to eliminate deposits on thecathode and to connect the cells electrically either in series orparallel.

WO 98/13304 describes the use of such an electrolytic cell in anapparatus to process a liquid, such as water. A liquid is supplied tothe cathode chamber only and part of the output from the cathode(catholyte) is recycled to the input of the anode chamber. This inputserves as the total supply to the anode chamber. In situations where notall of the solution output from the cathode chamber is recycled to theinput of the anode chamber, a proportion of the output from the cathodechamber is drained to waste, this proportion being measured by a flowmeter. A constant-voltage DC supply is applied between the anode and thecathode, and the pH and redox potential of the treated solutions aremeasured and maintained by controlling flow rates through the cell.

A method and apparatus for producing a sterilising solution is describedin GB 2316090, the subject matter of which is incorporated herein byreference, wherein a supply of softened water is generated by passingwater through an ion-exchange water softener. A saturated salt solution,generated by mixing softened water with salt, is passed through anelectrolytic cell to produce a sterilising solution, or used toregenerate the ion-exchange resin in the water softener.

However, all of the systems described above have drawbacks anddifficulties. For example, the variable factors, such as the degree ofelectrolysis in the electrolytic cells, the concentration of dissolvedsalts and minerals and the flow rates, the fluctuations in electricitysupply, ambient temperature and the variability of incoming watersupplies present a barrier to ensuring a consistent supply ofsterilising or, more correctly, biocidal solution. Thus in order toensure delivery of a biocidal solution, the electrochemical systemsdescribed all rely upon expert intervention to calibrate the cells atthe time of installation and to re-calibrate whenever the chemistry ofthe water supply changes to any significant degree.

As an illustration, the pH of the solution output from the anode chamber(anolyte) may be regulated by adjusting the flow rate of catholytedrained from the cathode chamber. This results in changes to the anolyteflow rate and consequently in changes to the electrochemistry takingplace in the electrolytic cell.

Also, the performance of all the above cells and methods is highlydependent on the alkalinity of the water and aqueous salt solutionsbeing treated. In Europe, for example, the alkalinity of potable watercan vary from very low (3-15 ppm CO₃ as CaCO₃) to very high (470 ppmCO₃) from one geographical region to another. This means that a cellwhich is calibrated to produce a biocidal solution of given compositionin a first geographical location may not produce the same biocidalsolution in a second location, making re-calibration necessary. This isa time-consuming and laborious task.

Minimising variation is important to ensure a supply of solution havingthe required properties, e.g. biocidal activity and pH, especially whenthorough sterilisation is required to maintain the health of apopulation.

Furthermore, it is important to be able to control to a fine degree thefinal composition of any biocidal solution produced, since the solutionmust have a high enough concentration of, say, available free chlorine(AFC) to be sufficiently biocidal, but not so high as to corrode orotherwise damage any equipment which is being sterilised. A stillfurther disadvantage of the apparatus described in the prior art is thatthey are prone to a high level of wastage. Up to half of the initialsupply of aqueous salt solution may be discarded after being passedthrough the cathode chamber. This is especially pertinent whereresources such as water are limited or costly.

In the Applicant's experience, none of the above systems is suited toproviding a wholly reliable or autonomous supply of biocidal solution.As will be readily appreciated, a “sterilising” solution which does notmeet the required level of biocidal efficacy carries a risk of allowingan instrument to spread infection. Moreover, the end user will not beable to detect by visual inspection alone whether the biocidal solutionfrom any one of these systems is within or outside specification.

SUMMARY OF THE INVENTION

Accordingly, the main object of the present invention is to provide asystem which delivers for use a biocidal solution only when it has thedesired properties, i.e. it is within specification. In this way, therisk of mistakenly using a solution which is not adequately biocidal canbe substantially eliminated.

There is also a need to provide a system which not only is capable ofproducing a biocidal solution in specification but also on demand.Moreover, there is a further need to provide a system which is able todeliver a biocidal solution in specification, on demand, at or close towhere the solution is to be used. In addition, there is a need toprovide a system which can operate irrespective of the parameters of thelocal source of input water. Ultimately, the Applicant has set out toachieve a system which is able to deliver biocidal solution inspecification, on demand, on site, anywhere.

To this end, and as a result of extensive trials and experiments, theApplicant has devised a system which, by virtue of various innovations,ensures that it will deliver biocidal solutions which are withinspecification. As will become apparent, the Applicant has also devised asystem which is able to produce in specification biocidal solution ondemand, on site, anywhere.

From one aspect, the invention resides in a method of operating anelectrochemical cell to produce an output solution having apredetermined level of available free chlorine, comprising applying asubstantially constant current across the cell between a cathode and ananode and passing a substantially constant throughput of chloride ionsthrough the cell.

In this regard, the Applicant has surprisingly found that by maintainingthese two constants, the output solution will have a predetermined levelof available free chlorine irrespective of other variables such as localwater hardness, alkalinity, pressure etc. In this way, reliance onexpert intervention whenever the water supply chemistry changessignificantly may be substantially reduced or even eliminated entirely.

Expressed in another way, the present invention resides in a method ofelectrochemical treatment of an aqueous solution in an electrolyticcell, wherein an output solution having a predetermined level ofavailable free chlorine is produced by applying a substantially constantcurrent across the cell between a cathode and an anode while passing asubstantially constant throughput of chloride ions through the cell.

The level of available free chlorine will be set according to thebiocidal properties which are required to be imparted to the outputsolution. The output solution will preferably be required to act as abiocide against a wide range of bacteria, fungi, viruses and spores. Anavailable free chlorine content of about 3 ppm to 300 ppm will generallyprovide biocidal properties for most envisaged applications. It willhowever be appreciated that biocidal efficacy is also dependant on pHand therefore that an appropriate balance must be achieved between pHand AFC in order to provide the desired level of bio-compatibility andmaterials compatibility. For example, the Applicant has found that alevel of available free chlorine of approximately 100-250 ppm at a pH ofbetween about 5 and 7 is particularly suitable for the application ofreprocessing heat sensitive medical instruments. Other applications,such as its use in non-medical environments, for example as in theprocessing of poultry and fish and general agricultural andpetrochemical uses, the breaking down of bacterial biofilm and watertreatment, may demand different levels of available free chlorine.

As will be discussed hereinafter, the Applicant has found that by usinga particular cell and flow arrangement, it is possible also to controlthe pH of the output solution. Where pH control is required, it ispreferable that the electrochemical cell comprises two chambersseparated by a separator, the first chamber comprising an anode chamberand the second comprising a cathode chamber.

It will be generally understood that the function of a separator in thecell is to isolate the solution in one chamber from the solution in theother chamber while allowing the migration of selected ions between thechambers and the term “separator” as used herein should be construedaccordingly. Semi-permeable diaphragms and ion-selective membranes arethe most common forms of known separators.

In an electrochemical reaction, it is known that the rate of reaction isgenerally directly proportional to current within certain limits of thecurrent. Therefore, the current (and thus the rate of oxidation ofchloride to chlorine) and flow of chloride through the cell may be setappropriately to produce an output solution having the predeterminedlevel of available free chlorine. The desired current will depend notonly on the type of cell being used, for example, the material fromwhich the electrodes are made and the various rare metals used toprovide active coatings on the electrodes, but also the size of cell,for example, for a cell having an anode surface area of approximately100 cm², an applied current between cathode and anode of 8 Amps isparticularly suitable.

In general, the voltage will change as the resistance of theelectrolytic cell changes, for example, through deposition of scale inthe separator. Accordingly, if the voltage, but not the current, is keptconstant, the resistance in the cell will increase as the cell is used.In accordance with Ohms Law, the current will drop and therefore theconcentration of available free chlorine in the output solution willfall. This will result in an output solution which may not havesufficient available free chlorine to enable it to act as a biocide.Therefore, previous systems, such as that described in WO 98/13304,which have relied on a constant voltage across the cell are not alwaysable to produce a predictable level of available free chlorine. In otherwords, with constant voltage systems, the biocidal properties of theoutput solution cannot be guaranteed.

However, the Applicant has appreciated that under conditions of constantcurrent, the voltage across the electrolytic cell can be monitoredusefully to provide an indicator of other parameters, such as theperformance of the apparatus used to carry out the method. For example,as described above, the voltage across the cell will change as theseparator becomes plugged with deposits. Also, the voltage will alter asthe active coating on the electrodes decreases or a catastrophic event,such as rupture of the separator, occurs in the cell. In this way,monitoring of the voltage provides a means for predicting the longevityof the cell.

In order to achieve a constant chloride ion throughput, it isadvantageous to control the flux of chloride ions into the cell. Forconvenience, the chloride ions are supplied to the cell as a salinesolution. Therefore, the throughput of chloride ions through the cellmay be determined by controlling salinity and flow rate.

While it is envisaged that the saline solution may be of variableconcentration and therefore the flow rate must also be varied to providea constant chloride feed into the cell, by supplying the saline solutionat a substantially constant concentration, only relatively minor changesin flow rate need be made to provide the constant chloride ionthroughput.

Desirably, the substantially constant chloride ion throughput isachieved by providing a substantially constant salinity at asubstantially constant flow rate. In this way, the quality of the input,in terms of the desired concentration of chloride ions supplied to thecell, is easier to predict and control. A further advantage of aiming toprovide a constant salinity is that, should any significant changes insalinity be detected, this may be correctly attributed to an error suchas a malfunction of the apparatus or loss of the saline supply. In thesecircumstances, a failsafe mechanism which is preferably incorporated inthe system can operate to prevent output solution which does not meetthe desired level of biocidal efficacy, i.e. is out of specification,from being dispensed.

Constant salinity may be achieved by a variety of means, for example bydissolving a known quantity of salt in a known quantity of water.However, this requires a level of skill as well as a knowledge of localwater parameters to ensure that the exact amount of salt is added toproduce the desired salinity. Accordingly, the Applicant has devised amethod of producing a desired salinity which avoids these drawbacks.

In particular, and after much experimentation, the Applicant has foundthat the chloride input to the cell can be more easily regulated byproducing a saline solution from a saturated salt solution, or at leasta concentrated salt solution, which is then diluted to the requireddegree. Preferably, the concentrated salt solution is obtained by addingan excess of salt to water, with further water and/or salt beingintroduced as required.

More especially, by dispersing discrete volumes of concentrated saltsolution into a flow of diluent, the cell can be fed with asubstantially constant chloride concentration at a constant rate. TheApplicant has found that a saline solution diluted to a concentration ofless than 1% w/vol, more preferably in the region of 0.3%, isparticularly suitable. The preferred concentration will however bedetermined according to a number of factors specific to the electrolyticcell being used and the type of output solution desired.

It is preferred if the concentrated salt solution is pulse fed into aflow of diluent water, for example by means such as a peristaltic pump.In this way, each pulse is directed to deliver a known quantity ofconcentrated salt solution. Accordingly, as the concentrated saltsolution becomes more dilute, for example as the supply of salt isdepleted, the pulsing rate of the concentrated salt solution into thewater flow is increased.

The Applicant has found that benefits are achieved by periodicallyallowing the concentrated salt solution to become increasingly dilute.By such means, deposits of crystalline salt in the apparatus in whichthe concentrated salt solution is prepared are reduced.

After the concentrated salt solution has been dispersed in the water, itis further preferred that the salinity is confirmed before entry intothe cell, for example, by measuring the conductivity of the salinesolution. Advantageously, this is achieved by way of a conductivityprobe.

If the conductivity does not fall within the desired range, means foradjusting the salinity to return the conductivity to within the desiredrange may be provided. This can be achieved by increasing or decreasingthe pulse rate to raise or lower the level of concentrated salt solutionbeing fed into the water flow. Alternatively or in addition, means toadjust the flow rate of the water to the cell may be provided. In thisway, namely adjustment of the pulses and/or the flow rate, fluctuationsin the chloride concentration reaching the cell may be substantiallyevened out.

Simply pulse feeding discrete volumes of concentrated salt solution intoa flow of water diluent can result in a stream of saline solution ofvariable chloride concentration. For example, the saline concentrationmay have peaks and troughs along the stream corresponding to the pulsesof concentrated salt solution. If the saline solution is not asubstantially uniform mix, the conductivity of the solution, if measuredprior to entry into the cell, may not be representative of the actualchloride ion content of the saline solution as a whole. Accordingly, itis another object of the invention to provide a means for achievingrapid and effective mixing of the concentrated salt solution in thewater diluent.

To this end, the present invention also resides in a method of mixingmiscible liquids, comprising dispersing one liquid from a pulsed sourceinto another liquid supplied as a continuous stream, wherein the pulsedliquid is discharged and dispersed in the continuous stream through aplurality of apertures along the flow path to produce a flow ofuniformly mixed liquids.

By dispersing a pulsed liquid into another liquid flow through a seriesof apertures, it is possible to minimise fluctuations in concentrationand produce a substantially homogenous mixture.

Expressed in another way, the invention resides in a method of combiningat least two liquids, wherein a first liquid is supplied as a continuousstream and a second liquid miscible with the first liquid is suppliedfrom a dispenser into which the second liquid is pulsed and dispersedinto the supply stream of the first liquid through a plurality ofapertures in the dispenser thereby to produce a continuous homogeneousstream of first and second liquids.

More particularly, the invention comprises a method of combining atleast two liquids, wherein a continuous stream of a first liquid iscaused to flow through a conduit and a second liquid miscible with thefirst liquid is pulsed into a liquid dispenser located in the conduitand dispersed into the flow of the first liquid through a plurality ofapertures provided in the dispenser thereby to produce a continuousstream comprising a homogeneous mixture of first and second liquids.

Preferably the dispenser is substantially elongate, for example in theform of a length of tube having an external diameter less than theinternal diameter of the conduit, which itself may comprise a tube, andhas a closed end and an open, feed end. The volume of second liquidwhich is pulsed into the first liquid will be determined by the volumeof the dispenser. Moreover, the length and diameter of the dispenser maybe selected to achieve homogeneity of the mixed first and second liquidsaccording to the preferred pulsing rate and pulsed volume of the secondliquid.

For maximum effect, the apertures are preferably arranged substantiallyevenly both longitudinally and circumferentially of the dispenser.Conveniently the apertures comprise perforations and their size may bevaried depending on the nature of the first and second liquids involved,for example, in accordance with their viscosities.

By means of the aforementioned mixing method, the Applicant has foundthat it is able to deliver a fixed volume of concentrated salt solutionand simultaneously to disperse the said volume in water to produce acontinuous flow of uniformly mixed saline solution. By such means, trulyrepresentative conductivity measurements of the saline solution can bemade prior to entering the cell.

The final concentration of the mixed saline solution will be determinedby the volume of the dispenser, the pulsing rate of concentrated saltsolution into the dispenser and the flow rate of the water diluent. Forexample, the Applicant has calculated that, to produce a 0.3% salinesolution from a concentrated salt solution of about 12% w/vol, thedispenser should have a length in excess of about 0.19 m. Ideally, theperforations in the dispenser have an inner diameter of approximately 1mm, and that about ten perforations are sufficient for this application.

In a typical system practising the method of the invention, theconcentrated salt solution is preferably pulsed at a rate of betweenabout 1 to 5 liters per hour and the water diluent is supplied at a rateof between about 150 to 250 liters per hour to achieve the targetchloride concentration from the dispenser.

As will be appreciated, the flow rate of the water diluent will belargely determined by the pressure of the supply and may be controlledby its back pressure. Alternatively, or in addition thereto, the waterpressure may be regulated by causing the water to flow through one ormore flow restrictors, for example, in the form of one or more orifices,provided along the diluent flow path. Ideally, the size of the or eachorifice can be increased or decreased to adjust the flow. In this way,the aperture size of the or each orifice may be varied appropriately toregulate the pump output to a constant flow.

Having now achieved the required salinity, for example by theaforementioned mixing method, the actual flow of saline into the cell isthen preferably regulated by means of one or more flow regulators beforeentry into the cell. Ideally, the saline supply is split such that aportion is fed to the chamber including the anode, and the remainder isfed to the chamber including the cathode. Advantageously, the catholytefeed includes its own regulator.

Preferably, a larger proportion of the saline solution is fed to theanode chamber than is fed to the cathode chamber. The Applicant hasfound that a ratio of at least 80% to 20% fed to the anode and cathodechambers respectively is particularly suitable to produce a biocidalsolution from the anolyte. Moreover, in this way, the amount of usefulproduct is maximised whilst the amount of waste is minimised. Aparticularly preferred feed ratio is 88% saline solution to the anode to12% to the cathode.

This parallel input to the two chambers of the cell represents a furtherdeparture from the prior art which describes the use of series inputs,first to the cathode chamber and then the anode chamber. Such a dualparallel pass allows for even greater control and regulation of thecomposition of the output solution, thereby substantially ensuring thatthe final product has the required biocidal properties.

As will be understood, the electrochemical process may be achieved by aplurality of electrolytic cells connected in series electrically, and inparallel hydraulically. Accordingly, references herein to anelectrolytic cell should be construed as including a plurality of suchcells.

In summary, by applying a constant current across the cell and aconstant throughput of chloride ions through the cell as hereinbeforedescribed, it is possible to produce an output solution from the anodechamber which has sufficient available free chlorine to impart biocidalproperties to the solution.

Accordingly, the invention may alternatively be expressed as a method ofproducing a biocidal solution whereby water and aqueous salt solutionare mixed to provide a saline solution of constant concentration whichis passed through an electrolytic cell at a constant flow rate, and aconstant current passed through the saline solution in the cell toproduce an output solution having a desired level of available freechlorine.

As previously mentioned, the biocidal efficacy of the output solution,and in particular the anolyte which provides the source of availablefree chlorine, is strongly dependant on its pH. It is thereforeadvantageous to tailor the final pH of the anolyte to suit the desiredend use. For example, and as described in the Applicant's copendingUnited Kingdom patent application no. 9919951.5, a pH of about 5 issuitable for use in treating venous leg ulcers to reduce bacterialinfection, while a pH of between 5 and 7 is more suitable for use in thedisinfection and sterilisation of heat-sensitive endoscopes. To avoiddeterioration of pH-sensitive material, a neutral pH of about 7 may beappropriate. Accordingly, the method of the invention preferably furtherincludes adjusting the pH of the output solution which in turn requiresthe pH of the anolyte to be monitored.

Altering the pH of the output solution may conveniently be achieved byfeeding at least part of the catholyte to the anolyte. The catholyte maybe fed to the anolyte either upstream or downstream of the cell.Preferably, at least part of the catholyte is recirculated into theanode chamber. The proportion of catholyte which is fed to the anolytedepends on the final pH required and may be determined by routineinvestigation. Accordingly, the method of the invention preferably alsoincludes regulating the proportion of catholyte fed to the anolyte.

A further benefit achieved by recycling a proportion of the catholyte isthe reduction of the actual amount of catholyte which goes to waste.This is especially desirable as the catholyte waste contains sodiumhydroxide. By means of catholyte recirculation, the Applicant hasachieved a reduction in waste to less than 10% of the total liquid fedinto the cell and even this level of waste can be further substantiallyreduced. As will be appreciated, cutting the amount of waste liquid tosuch levels provides a considerable advantage where resources such aswater are at a premium. Moreover, by using a by-product of the processto control the pH, the external supply of another process componentwhich may otherwise be required to control pH may be avoided.

Accordingly, and from yet another aspect of the present invention, thereis provided a method of electrochemically treating a supply of salinesolution in an electrolytic cell having an anode chamber and a cathodechamber separated by a separator, the anode and cathode chambersrespectively being provided with an anode and a cathode, and eachchamber having input and output lines for the solution being treated,wherein:

i) saline solution is supplied to the anode and cathode chambers by wayof their respective input lines, at least the cathode chamber input linebeing provided with a flow regulator, and output by way of theirrespective output lines;

ii) a substantially constant current is caused to flow between the anodeand the cathode; and

iii) a proportion of the solution output from the cathode chamber isrecirculated to an input or output line of the anode chamber by way of arecirculation line.

This and other aspects of the invention are also disclosed in thespecification of United Kingdom patent application no. 9918458.2, thesubject matter of which is incorporated herein by reference.

It is believed that the output solution owes its biocidal properties tothe presence of available free chlorine in the form of oxidising speciesincluding hypochlorous acid (HOCl) and sodium hypochlorite (NaOCl⁻).Such reactive species have a finite life and so, while the pH of theoutput solution will usually stay constant over time, its biocidalefficacy will decrease with age.

While the output solution will therefore have the desired biocidalefficacy on production, there is a risk that it will fall outside therequired specification if stored for any period of time rather thanbeing used immediately. As a further safeguard therefore, the method ofthe present invention further includes disposing of the output solutionafter a period of time. In this regard, the Applicant has found that theoutput solution generally maintains a sufficient level of biocidalefficacy for a period of more than twenty four hours. However, to becertain that the output solution is sufficiently biocidal, the methodincludes disposing of unused output solution if not used within abouttwenty four hours of its production.

The Applicant has found that dilution of the output solution produces abacteria-free water which retains a measure of the biocidal propertiesof the output solution. Such bacteria-free water has a number ofapplications including the rinsing of heat-sensitive medical instrumentsfollowing disinfection or sterilisation and the rinsing of glassware insterile laboratory or pharmaceutical manufacture applications. Newstandards are continually being applied to such rinsing agents and theApplicant considers that the properties of the output solution producedin accordance with the present invention, when diluted, provide aneffective bacteria-free water (hereinafter referred to as bacteria-freerinse water merely to distinguish it from the neat output solution),which exceeds the required standards. Advantageously, therefore, themethod according to the invention further includes the step of dilutingthe output solution to produce a bacteria-free rinse water.

The Applicant believes that, to ensure the biocidal efficacy of thebacteria-free rinse water, the output solution used to make up the rinsewater is preferably not more than about three hours old. In accordancewith various failsafe provisions in the preferred method of theinvention, any output solution which is detected to fall outside therequired specification is generally discharged to waste regardless ofits age. However, for the purposes of bacteria-free rinse water, even“in specification” output solution will not be used to generatebacteria-free rinse water if it is more than the desired maximum age.

In order that only the most freshly produced output solution is used toprepare bacteria-free rinse water, it is preferred that output solutionemerging from the cell is fed into an intermediate holding location, forexample in the form of a first holding means, from which output solutioncan be drawn for preparation of bacteria-free rinse water. Any outputsolution which is not used to prepare bacteria-free rinse water ispassed into a further holding location, for example in the form ofstorage means.

For convenience, the output solution may be initially held in theintermediate holding location and from where it is permitted to overflowinto the further holding location after a predetermined volume of outputsolution has been produced. More conveniently, the further holdinglocation may be located beneath the intermediate holding location suchthat output solution spills directly into it from the intermediateholding location. Ideally, the intermediate holding means comprises aweir tank from which output solution may overflow into a storage tank.To save space and to reduce any risk of external contamination, the weirtank is preferably housed inside the storage tank.

The weir tank provides an ideal location at which to check or confirmthat the output solution has the desired parameters. Thus, the weir tankis preferably provided with means to measure redox potential and pH. Ifthe measurement shows that the output solution entering the weir tankfalls outside the required specification, the entire contents of theweir tank are preferably diverted to waste thereby avoidingcontamination of the second, storage tank and its contents, and avoidingthe risk of preparing rinse water from a bad solution. In any event, itis desirable that the contents of the storage tank are disposed of if itcontains output solution which has been held for more than about twentyfour hours. Furthermore, to ensure that bad output solution entering theweir tank does not accidentally spill over into the storage tank, it isdesirable that the flow rate of output solution to waste is faster thanits flow rate, or rather its overflow rate, into the storage tank.

The intermediate holding location is preferably open to the atmospherethereby to reduce the back pressure that may be exerted on the cell andto which the cell is known to be sensitive. In this regard, a weir tankagain provides a particularly suitable option.

It is preferable that the intermediate holding means such as the weirtank is of sufficient capacity to meet a typical demand forbacteria-free rinse water from output solution, whilst at the same timeminimising the volume which would be wasted should the output solutionfall out of specification. As will be understood, the ideal capacitywill depend on the desired output of the machine.

A further advantage of using a tank, such as a weir tank, as theintermediate holding means, is that it provides a known capacity intowhich additional reagents may be added to the output solution containedtherein. For example, it is highly desirable to add a corrosioninhibitor to the output solution to prevent corrosion, not only of theapparatus used to generate and dispense output solution, but also theitems exposed to biocidal solution during sterilisation anddisinfection.

Accordingly, the method according to the invention preferably furtherincludes the step of adding a corrosion inhibitor to the outputsolution. More preferably, the corrosion inhibitor is added after theoutput solution has been confirmed to have the desired parameters andprior to dispensing. In this way, corrosion of any apparatus orequipment which is contacted by the output solution is reduced orsubstantially eliminated.

It is however important that any corrosion inhibitor added to the outputsolution does not significantly affect the biocidal properties.Moreover, it is also important that the non-toxic, non-hazardousproperties of the biocidal solution are not compromised. In this regard,a preferred corrosion inhibitor comprises a combination of apolyphosphate with a molybdate, more preferably a mixture of sodiumhexametaphosphate and sodium molybdate.

For convenience, it is desirable to be able to produce output solutionon demand, at or close to where the solution is to be used, such as in ahospital. In this way, the need to transport output solution, forexample in bottles, to where the solution is to be used may be avoided.In other words, the method of the invention preferably allows for theoutput solution to be dispensed directly for use.

While the output solution may be dispensed directly for use from thecell if there is an immediate need for the solution, it is alsodesirable to allow for output solution and, if produced, bacteria-freerinse water to be stored until required. As will be appreciated, thecapacity of any such storage means will generally be determinedaccording to the required end use and level of demand. Clearly, thesestorage means will generally have a greater capacity than theintermediate holding means or weir tank. For example, the Applicant hasfound that a storage capacity of about 90 liters is sufficient to supplya demand from several typical washer-disinfecter machines which requirefilling in the shortest possible time. Such washer-disinfecter machinesare frequently used for the sterilisation of medical instruments, suchas endoscopes. Furthermore, the volume of output solution produced willbe determined by the number of electrolytic cells utilised and thereforethe capacity of the storage means should ideally be sufficient to copewith this volume. In this regard, it has been found that eight cellsconnected hydraulically in parallel are together capable of a productionvolume of approximately 200 liters per hour.

A still further advantage is seen if the volume of output solutionand/or bacteria-free rinse water stored is sufficient to facilitate therequired dispersion of any additives, such as corrosion inhibitor, tothe solutions.

As a further safety mechanism, it is highly desirable for the systemproducing the output solution to be self-monitoring. In this way, shouldany parameters, such as process or materials parameters, be detected tofall outside desired values, or any rapid or unexpected changes bedetected, the system can be alerted. For example, measurements mayindicate that more raw materials are required or that there is a faultin the production process. By incorporating self-monitoring inconjunction with an alert mechanism, the risk of generating a volume ofoutput solution which is out of specification may be substantiallyreduced.

Advantageously, the system incorporates a self-alert mechanism which ispreferably adapted to trigger a self-correction action and/or to notifya user of the system that there is a fault or demand. However,auto-correction, where possible, is preferred before an alarm is raised.For example, self-adjustment of flow rates may be all that is requiredto cope with fluctuations in local water pressure and alkalinity,whereas a disruption to the supply of input water may not necessarily besusceptible to auto-correction. As a yet further safety precaution orfailsafe, it is preferred that production of output solution be stoppedshould self-correction not be possible or there be no response to analarm. In this way, the possibility of dispensing output solution whichfails to meet the desired parameters can be substantially avoided.

From another aspect, it is desirable if the system allows a user tointeract with the production process, such as to obtain information onthe performance of the system. Such interaction ideally allows the userto confirm that the production process is functioning properly and, ifnot, provides the user with guidance as to what action(s) can or shouldbe taken to remedy any faults or deficiencies. Of course, any systemfaults or deficiencies which are not susceptible to auto-correction arelikely to have been brought to a user's attention already by way of analarm. In circumstances where faults or deficiencies are not easilyremedied by the user, or where an indication is provided that the systemwill require servicing, the user may be prompted to call an expert.

However, it is useful to permit a user to interact with the system otherthan under alarm conditions, for instance to enable the user toascertain whether or not there is sufficient output solution and/orbacteria-free rinse water to meet anticipated demand, to advise the userto wait for sufficient output solution to be generated, or to add saltand/or water. In addition, the user may be provided with information asto cell performance and/or its predicted lifespan thereby enabling thecell to be replaced at a convenient time, rather than having to react toa cell failure.

It will be appreciated that the user interface may be governed bycomputerised means, for example, with provision of suitable firmware andsoftware. Typically, the system may be microprocessor controlled withthe interface ideally provided through a display, keypad and/or printermeans to provide on-site control.

While it is preferred that the process by which output solution isproduced is self-adjusting, in the event that a fault cannot berectified by self-adjustment, it is advantageous if self-diagnosticmeans are provided to identify where possible the nature of the fault.Accordingly, it is preferred that the system of the invention furtherincludes a service interface, through which an engineer may gain accessto diagnostic information prior to taking remedial action. As with theuser interface, the service interface will also be governed by suitablesoftware.

For flexibility and convenience, it is preferred that service interfacebe accessed either on-site or remotely via a modem or the Internet. Anadvantage of permitting remote access is that an engineer may check theapparatus on a regular basis without having to travel to the site of theapparatus. This is of considerable benefit when the system has beeninstalled in a far location.

The service interface may also be adapted to provide a history of theproduction process, for example how the production process hasfunctioned over a period of time and hence to ascertain the remaininglife expectancy of a particular component. Also the consumption ofoutput solution can be monitored periodically. Different levels ofaccess to the service interface may be provided, for example access tothe production process history may be restricted to engineeringpersonnel.

A further advantage is seen if a system engineer is provided with meansto alter operating parameters remotely where possible, thus reducing thenecessity for the engineer to attend the system if the process requiresonly minor adjustment. Also, this enables the engineer to monitor thesystem to keep it working smoothly. Indeed, by facilitating remoteaccess, it is possible for an engineer to make adjustments to the systemwell before any alert mechanism is triggered. In such a way,intervention by the user can be kept to a minimum. Indeed, under typicalconditions, a user may be required only to feed the system with salt atappropriate intervals, as any other controls or adjustments are made bythe system itself or remotely through the service interface.

If remote access is provided via the Internet, for example, it isenvisaged that such access may also include means by which the systemcan alert an engineer of a problem, for example, by e-mail, so that theapparatus may be attended to before a potentially more serious faultoccurs. It is also possible to alert an engineer by fax, short messageservice (SMS) or other such means. All of these service interfacefeatures can help to reduce downtime of the apparatus and facilitatesiting of apparatus in diverse locations.

All of the aforementioned features contribute to providing a systemwhich delivers, for use, an output solution which has sufficientavailable free chlorine to impart biocidal properties to the solution.In other words, and by means of the various self-checking and alertmechanisms, it will be appreciated that the system is adapted to preventoutput solution which is not within specification from being dispensed.

From another aspect, the present invention resides in apparatus forproducing an output solution having a predetermined level of availablefree chlorine comprising an electrolytic cell, means for passing asaline solution having a substantially constant chloride ionconcentration through the cell, means for applying a substantiallyconstant current across the cell and means for dispensing outputsolution from the cell.

As will be appreciated, by means of such apparatus, it is possible toproduce an output solution having biocidal properties almost anywherewhere there is a supply of process water, salt and electricity.

Preferably, the apparatus is provided with water input means including asupply tank for storing and dispensing process water. Since pressurefrom a local water source may vary, such a supply tank compensates forany fluctuations and thereby acts as a hydraulic capacitor.Conveniently, the supply tank is of sufficient capacity to cope with anysuch fluctuations. A further advantage of storing process water is thatthe supply tank provides a ‘reserve’ supply of water to the process,should the local supply be disrupted for any reason.

The means for generating saline solution having a substantially constantchloride concentration preferably comprises salt input means, waterinput means, means for dissolving salt in water to produce aconcentrated salt solution, means for mixing and diluting theconcentrated salt solution to a desired concentration and means forfeeding the resulting saline solution to the electrolytic cell at aregulated rate.

It is preferred that the salt input means comprises a chute whichideally holds a known quantity when filled to a predetermined level andwhich transfers salt to a concentrated salt solution make-up tank. Forexample, the Applicant has found that about 6 kg of salt is convenientbecause this corresponds to an easily-handled amount and, under typicaloperating conditions, provides an adequate supply of salt to theapparatus for a period of, say, two days.

Salt is generally dissolved in water from the input means to produce aconcentrated salt solution in the make-up tank. Following an input offresh salt, the solution may at least initially be a saturated saltsolution. A level detector may be provided in the make-up tank toprovide an indication when the salt level is insufficient to produce aconcentrated saturated solution. Such a detector is preferably linked toan alert mechanism, such as a visual or audible alarm, which isactivated to advise a user that more salt is required. Moreover, theapparatus is preferably provided with a mechanism designed to haltproduction of output solution if the alarm is not responded to within aspecified time period.

The concentrated salt solution is diluted with process water to thedesired concentration. As previously described, this is preferablyachieved by pulse feeding concentrated salt solution, for example usinga peristaltic pump, from the make-up tank into a flow of process watersupplied by the supply tank via a dispenser. The dispenser may beprovided with a series of apertures thereby ensuring that the pulses ofconcentrated salt solution are substantially evenly dispersed in theprocess water. By these means, a saline solution of a desiredconcentration may be produced.

Moreover, to confirm the concentration of chloride ions in the resultingsaline solution, a conductivity probe or any other suitable measuringmeans is conveniently provided before the solution enters the cell. Ifthe chloride ion concentration as measured does not fall within thedesired range, the pumping rate of the saturated salt solution and/orthe process water may be adjusted by feedback means from theconductivity probe. Additionally, one or more flow regulators may beprovided as a fine tuning mechanism for the saline solution entering thecell.

Having confirmed the conductivity and regulated the flow accordingly,the saline solution is fed into an electrolytic cell. Electrolytic cellsfor producing biocidal solutions are of course known and preferablycomprise co-axial cylindrical and rod electrodes separated by aseparator, such as a semi-permeable or ion-selective membrane. Usuallythe electrodes are made of titanium, and the anode is provided with anactive metal oxide coating. Generally, the cylindrical electrode isconnected to the positive output of a current source, and the rodelectrode is connected to the negative output, but a reversal of thisarrangement is also known.

While such known cells may be used in the system according to thepresent invention, the Applicant has developed a new cell which isparticularly suitable. From another aspect therefore, the presentinvention comprises an electrolytic cell having an anode chamber and acathode chamber separated by a separator, the anode and cathode chambersrespectively being provided with an anode and a cathode, each chamberhaving at least one input and output, wherein the separator is in theform of a semi-permeable membrane comprising an aluminium oxide basedceramic containing zirconium oxide and yttrium oxide.

As will be understood, it is a desired function of the separator that itbe sufficiently permeable to permit an adequate flow of solution betweenthe two chambers to give an acceptable electrical resistance while beingsufficiently non-permeable to prevent gross mixing of the anolyte andcatholyte solutions. In this regard, the Applicant has found that aceramic comprising up to 20% zirconium oxide and up to 2% yttrium oxidesatisfies this function. More desirably, the ceramic consistsessentially of 80% aluminium oxide, 18.5% zirconium oxide and 1.5%yttrium oxide. The porosity of the ceramic is preferably within therange of 50-70% and the pore size between 0.3-0.5 microns. Furthermore,the ceramic preferably has a wall thickness of 0.3-1.0 mm.

A particularly suitable ceramic membrane composition and its method ofmanufacture is disclosed in the specification of the Applicant's ownco-pending United Kingdom patent application no. 9914396.8, the contentsof which are herein incorporated by reference.

Alternative separation means may be provided by an ion-selectivemembrane comprising a perfluorinated hydrocarbon containing sulfonateionic groups having channels which permit the passage of cations onlythrough the membrane, for example, the membranes sold by DuPont underthe trade mark Nafion®

As with the known cells referred to, the electrolytic celladvantageously comprises co-axially arranged cylindrical and rodelectrodes, preferably with the cylindrical electrode forming the anodeand the rod electrode forming the cathode. Preferably, the cathode has auniform cross-section along its effective length.

Moreover, the anode is preferably formed from titanium, and desirablyincludes an electrocatalytic (active) coating for the oxidation ofchloride ions, for example mixtures of any or all of ruthenium oxide,iridium oxide, and titanium oxide.

The electrolytic cell may alternatively be of a filter-press typedesign, with flat electrodes separated by an ion-selective membrane,such as that previously referred to and sold under the trade markNafion®. However, such a cell is less preferred than the cylindrical androd electrode type.

As previously described, the electrolytic cell preferably includesseparated anode and cathode chambers, and saline solution is fed intoboth chambers simultaneously with a constant current applied between theelectrodes. Output solution is passed from the anode chamber todispensing means while the catholyte is either directed to waste or aportion thereof recirculated into the anode chamber.

The dispensing means preferably comprises one or more storage tanks.However, in view of the desirability to use only the output solutionwhich has been produced within a preferred time period, as describedabove, the Applicant has devised an arrangement of storage tanks whichallows for this. Accordingly, the output solution is preferably fed intoan intermediate holding tank, such as a weir tank, before it istransferred to one or more main storage tanks.

In order to confirm that output solution entering the intermediate tankhas the desired characteristics, quality control means such as redox andpH probes maybe incorporated to provide data on the output solution asit enters the tank. The intermediate tank may be further provided withdischarge means to divert output solution, which does not fall withinthe specification, to waste. Other means may also be provided to feedin-specification output solution from the intermediate tank to a storagetank from which the solution may be dispensed for use and/or dispensedto a yet further storage tank where it may be diluted to producebacteria-free rinse water.

When charged to a predetermined level and having had its redox potentialand pH confirmed as falling within specification, the weir tank allowsoutput solution to overflow into the main storage tank.

As will be appreciated, gases such as hydrogen and chlorine aregenerated by the electrochemical reaction in the cell. Since these gasesare potentially dangerous and the chlorine itself malodorous, it ishighly desirable that these gases be removed from the output solutionbefore it is dispensed for use. Preferably, the gases are vented fromthe output solution through one or more filters. Ideally, a filter, suchas a carbon filter, is located to catch such gases from the outputsolution in the weir and/or other storage tanks, such as thebacteria-free rinse water storage tank.

For most applications, the apparatus as described is preferably housedin a self-contained unit. However, it may alternatively be provided in amodular format, for example so that it may be constructed on site withinthe restrictions of the available space. For ease of assembly andmaintenance, and whether a self-contained or modular format, connectionsbetween the components of the apparatus are most conveniently providedin the form of rigid pipes. The pipes may be connected to the componentsand/or each other by means of universal joints or threaded connections.Accordingly, when one or other of the components is replaced or removedfor maintenance, the need to use tools such as spanners may besubstantially avoided.

In assembling the individual components to form the apparatus, theApplicant has done far more than simply arranging the components in sucha way as to accommodate them in a convenient housing. In particular, theApplicant has expended much time and effort to achieve an assembly whichprovides both practical and technical benefits. For example, theApplicant has arranged the components so that the various pumps arelocated at a low level within the apparatus thereby not only lendingstability to the apparatus but also helping reduce vibration of theapparatus caused by operation of the pumps. Similarly, location of theprocess water supply tanks and the concentrated salt solution make-uptank at a low level provides further stability. Low level location ofthe saturated salt solution make-up tank is also particularly convenientas it provides for feeding from the salt chute at a comfortable height.

Furthermore, it has been found that by locating the electrolytic cell ata level which is higher than the aforementioned input tanks, backpressure on the cell is substantially avoided. Moreover, since theelectrolytic cell is at a relatively high level, this makes it possiblefor output solution to be transferred to one or more storage tanks alsoat a high level. In this way, dispensing of the output solution from theor each storage tank, either as neat biocidal solution or asbacteria-free rinse water can be achieved by gravitational feed.However, where it is required to dispense a large volume of solutionover a short period of time, for example, as required to fill awasher-disinfecter machine, gravitational feed alone may not besufficient and so it is advantageous if the output lines also includepumping means.

As will be appreciated, it is highly desirable for the carbon filter tobe located at a high level with respect to the apparatus. In this way,it is possible to maximise the collection of gases generated by theelectrochemical reaction and to minimise the risk of exposing personnelto those gases.

Means to detect any leakage of liquids from the apparatus may also beincluded, such detection means advantageously being in communicationwith the user and or service interface so that remedial action may bepromptly taken. Ideally, the user/service interface will provideinformation as to the source of the leak. Leak detection means may beconveniently located in a drip tray positioned at the base of theapparatus.

In order that the system is not compromised through lack of cleanlinessin the apparatus, it is desirable that it be self-cleaning, preferablyby means of an automatic self-cleaning cycle. In this respect, it isadvantageous if the self-cleaning cycle is designed to ensure that atleast those parts of the apparatus which may come in contact with outputsolution are cleaned. Effectively, this means that various pipes,valves, pumps, probes, connectors and storage tanks are required to becleaned. Since the apparatus is adapted to generate an output solutionhaving biocidal properties, this solution is ideal to carry out thecleaning. In this way, the apparatus is also disinfected and sterilised.

Accordingly, the method of the invention further includes an automaticself-cleaning step whereby output solution is periodically passedthrough substantially the entire apparatus. As will be appreciated,because the system operates in such a way as to prevent output solutionwhich is not within specification from being dispensed, only outputsolution which has the required biocidal properties may be used in thecleaning step.

To ensure that all surfaces of the storage tanks are contacted by theoutput solution during the cleaning process, it is advantageous if thesolution is introduced into each tanks by way of a spray bar.

In addition, to minimise downtime of the system, it is preferred thatthe operation of the self-cleaning cycle takes place at a time when thesolution is least likely to be demanded, for example, at night.

It is a preferred object of the invention that the system can operateirrespective of local conditions. Since the nature of water supplies mayvary enormously between locations, for example its supply pressure andtemperature, hardness, pH and microbial count, it is desired to providea system which can be adjusted to perform irrespective of theseparameters. Accordingly, it is advantageous if the apparatus includesmeans to compensate for parameters which fall outside the preferredoperating range.

For example, variations in water supply pressure can be compensated forby means of the process water supply tank. A high microbial count can bereduced by suitable filtration before the water is allowed to enter thesupply tanks, this is especially pertinent to use of apparatus indeveloping countries where the water may be of poorer quality.

Variations in pH of the supply water may be compensated for by adjustingthe pH of the output solution to the required level by recirculating aproportion of the catholyte from the cathode chamber of the cell intothe anode chamber. This pH adjustment process and its advantages havebeen described above.

Water hardness may also affect the system, resulting in deposition ofmagnesium and calcium ions not only in the supply tanks, but moreseriously, in the cell itself. Such deposition may cause plugging of theseparator which increases the cell resistance and this in turn increasesthe wear on the cell. Life-expectancy and cell efficiency are therebyreduced. Also, the use of unsoftened water can make it more difficult tocontrol the pH of the anolyte. Accordingly, it is preferred toincorporate means for substantially removing the hardness ions from thewater supply or at least reducing the amount of such ions before itpasses into the supply tanks. Such means maybe by way of a suitablewater softener, for example one containing a cation-exchange resin.

By virtue of the aforementioned features, the Applicant has devised anew system for generating an extremely effective non-toxic, biocidalsolution which acts against a wide variety of bacteria, fungi, virusesand spores and is suitable for many applications including disinfectionand cold sterilisation. In addition, the system can be operated andmaintained regardless of location and requires only water, electricityand salt to be put into effect. The system can be operated eithercontinuously or in response to demand and can be adjusted to produce asolution tailored for a particular end use. Moreover, because of thevarious failsafe means it incorporates, it is virtually impossible foran end user to be provided with a biocidal solution of inadequateefficacy.

In summary, the Applicant has invented a system which is not onlyadapted always to deliver biocidal solution which falls within thedesired specification, but also to deliver such solution on demand, onsite, anywhere.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments of the invention, will be better understood whenread in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown. In the drawings:

FIG. 1 shows an embodiment of the invention in schematic outline;

FIG. 2 is a detailed flow diagram of the invention as outlined in FIG.1;

FIG. 3 illustrates a dispenser in accordance with another aspect of theinvention; and

FIG. 4 shows an electrolytic cell for use in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, the schematic outline of the invention isbroken down into three main processing stages, namely an inputs andpre-processing stage, a production stage and a storage and dispensingstage. While referred to as stages, it will of course be appreciatedthat the process of the invention may be carried out continuously.

In the first (inputs and pre-processing) stage, there is an input ofpotable water which, for the purpose of generating saline solution foruse in the electrolytic cell, is first passed through a water softenerzone where excessive magnesium and calcium ions are removed. Thesoftened water is then passed into a process water buffer zone where itis held until required for use in the production of brine. Potable waterinput is also passed directly to the storage and dispensing stage foruse in the preparation of bacteria-free rinse water, but for thispurpose there is no need for the water to be softened prior to use.

The first stage also includes a salt (NaCl) input, usually of vacuumdried crystalline salt which is commercially produced to a consistentstandard, to a brine generation zone where a concentrated salt solutionis made up from the salt and the softened water obtained via the processwater buffer zone.

A further input is provided for additional agents, such as a corrosioninhibitor, used to condition output solution produced by the process.The conditioner is passed to a conditioner storage zone where it is helduntil required.

Turning to the second (production) stage, this comprises a constantsalinity subsystem in which a saline solution of substantially constantconcentration is produced by dilution of the brine from the brinegeneration zone with softened water from the process water buffer zoneto the desired concentration. The resulting saline solution is passedfrom the constant salinity subsystem to one or more electrolytic cells,each including cathode and anode chambers (not shown), and across whicha substantially constant electric current is applied. The appliedelectric current is maintained constant via an energy control andmonitoring zone.

Catholyte and anolyte are produced from the cathode and anode chambersrespectively as a result of the electrochemical treatment of the salinesolution in the cells. Anolyte and a portion of catholyte which is notrecirculated to the anode chamber are both dealt with in the third(storage and dispensing) stage. In particular, catholyte which is notrecirculated is directed to waste and anolyte, otherwise referred to asoutput solution, is passed to a buffer and quality subsystem. The outputsolution is tested in the buffer and quality subsystem and, if it failsto meet the quality standards, it is also directed to waste. If theoutput solution falls within specification, a quantity of conditioner,such as a corrosion inhibitor, is added to it in the buffer subsystemand the output solution is then permitted to pass either into an outputsolution storage zone from where it is subsequently dispensed for use orinto a rinse water subsystem.

Output solution directed to the rinse water subsystem is diluted withpotable water from the potable water input and is then passed to a rinsewater storage zone from where it is subsequently dispensed.

Provision is also made for discharging output solution from the outputsolution storage zone and rinse water from the rinse water storage zoneto waste.

Information on the various processing stages and the ability to interactwith the process is provided by means of a user interface and a serviceinterface. The service interface also provides for remote access to theprocess, enabling an off-site engineer to obtain information on and makeadjustments to the processing in each of the three stages.

There is also provided an autoclean subsystem to permit cleaning of thesystem, either at regular intervals or whenever convenient.

FIG. 2 is a flow diagram or “hydraulic map” showing in more detail theinvention already outlined in FIG. 1. Potable water is passed through anexternal water softener containing a cation exchange resin (not shown)thereby exchanging hardness ions of calcium and magnesium onto the resinand releasing sodium ions into the water.

Incoming softened process water is monitored by a sensor 10. The sensor10 ascertains whether the incoming water is at a temperature within therange under which the process can reasonably operate, namely betweenabout 5 and 35° C. Other parameters such as the incoming water'spressure, softness, alkalinity, pH, conductivity and microbial count canalso be monitored by the sensor 10 to establish that it falls withinacceptable levels for the process.

If the sensor 10 detects that the properties of the incoming softenedprocess water do not fall within acceptable limits required by thespecification, the water is diverted through a waste discharge manifold(not shown) to a drain via valve 12. On the other hand, if the incomingsoftened process water is in specification, it is allowed to flow intointernal process water tank 14 through inlet valve 16 or is diverted viainlet valve 18 to the concentrated salt make-up tank 20.

Buffer storage for the process water in the event of a temporaryinterruption in the water supply is provided by the process water tank14 having a large enough volume. Moreover, the tank 14 also hassufficient capacity in order to eliminate pressure fluctuations in thefluid supply to the electrolytic cells.

The process water tank 14 includes a plurality of level detectors formonitoring and controlling the process water level in it. Level detector22 is a safety device which is activated only when the process water inthe tank reaches a predetermined extra high level to stop the chargingof the tank with process water and raise an alarm. Another leveldetector 24 is activated when the level of liquid in the tank reaches apredetermined high level to stop further inlet water from entering thetank 14 by closing a valve 16. Water will begin to re-charge the tank 14after a predetermined time has elapsed below the high level. Leveldetector 26 is activated when the process water in the tank 14 reaches alow level to prevent production of output solution. The tank 14 alsoincludes a valve 28 which allows liquid to be drained. Furthermore, thetank 14 is designed to comply with local regulations, such as the classA air break requirements as required in the United Kingdom by BuildingRegulations Bylaw 11.

Concentrated salt solution is made-up and stored in a concentrated saltsolution make-up tank 20. To make up the concentrated salt solution,vacuum dried crystalline salt (BS998:1990) is added to the tank 20 via asalt chute 21 having a capacity which is able not only to accommodate atypical salt input of about 6 kg, but to tolerate an amount ofoverfilling sufficient to keep the system supplied for approximately 1to 2 days at a normal operation level.

To monitor liquid levels within the concentrated salt solution make-uptank 20, level detectors are also provided. Thus, level detector 30 is asafety device which is activated by an extra high level of liquid in thetank 20 and acts to close a valve 18 to prevent overfilling of the tank20 and to raise an alarm, but will not halt production of outputsolution. A level detector 32 is activated by a high level of liquid inthe tank 20 to stop further water filling the tank 20 by closing thevalve 18. A level detector 34 is activated by a low level of liquid inthe tank 20 and operates to open the valve 18 to charge the tank 20 withsoftened water. A low level detector 36 is activated by a very low levelof liquid in the tank 20 to halt production of output solution and toraise an alarm.

Softened water is fed through the valve 18 and automatically fills thetank 20 through a spray-bar 38 until the high level switch 32 isactivated. Salt in the tank 20 dissolves in the water to produce aconcentrated salt solution with the level of salt reducing as more saltis dissolved.

A further level detector 40, this time for the salt, is located towardsthe bottom of the tank 20. The salt level detector 40 is activated whenthe amount of salt in the tank 20 is depleted such that it isapproaching a level insufficient to produce a concentrated saltsolution. On activation, an alarm is raised which alerts an operatorthat more salt is required. The request to add salt is displayed on theuser interface (FIG. 1) and replenishment of the salt supply in the tank20 may be carried out manually by an operator or automatically through acontrol system. The user interface is operative to display a suitablemessage when sufficient salt has been added.

Finally, the tank 20 also includes a manual drain valve.

Concentrated salt solution from the salt make-up tank 20 is diluted withprocess water from the process water tank 14 to produce a salinesolution of substantially constant chloride ion concentration. In moredetail, process water is continuously pumped by process water pump 44through a valve 46 towards an electrolytic cell pack and concentratedsalt solution is pulse fed into the flow of process water via anadjustable speed peristaltic pump 48. The pulses of concentrated saltsolution are dispersed into the substantially continuous stream ofprocess water through a perforated tube 50 thereby evening out thepulses to produce a flow of saline solution of uniform concentration.

The flow rate of the resulting saline solution as it flows towards thecell pack is monitored by a flow meter 52 and if necessary is modulatedby a flow regulator in the form of an orifice plate 54. The flow rate ischanged simply by changing the size of the orifice in the plate.Different orifice plates may be chosen to suit site conditions.

Prior to entering the cell pack, the concentration of chloride ions inthe saline solution is checked by means of a conductivity sensor 56. Ifthe conductivity measurement indicates that the chloride ionconcentration has fallen below the desired level or has risen above it,the pulsing rate of the peristaltic pump 48 is increased or decreasedrespectively to alter the amount of chloride ions being dispersed intothe process water through the perforated tube 50 thereby compensatingfor the fall or rise in chloride ion concentration. The size of theaperture in the orifice plate 54 is also adjusted to regulate the flowof chloride ions into the cell pack. Adjustment of the pulsing rate andthe flow rate together provide a fine tuning means to ensure that thecell pack is supplied with a constant chloride ion throughput.

On the other hand, if the conductivity of the saline solution asmeasured by the conductivity sensor 56 falls outside a predeterminedrange such that it is not possible to adjust the pulsing rate and/orflow rate to bring the conductivity within the required range, and hencemake it virtually impossible for the cell pack to produce outputsolution having the desired level of available free chlorine, an alarmis raised and the flow of saline solution to the cells is ceased pendingrectification of the problem.

If the saline solution already provides or can be adjusted to providethe requisite throughput of chloride ions, it is split into two streams58, 60 before being fed through the cell pack. Typically the cell packconsists of eight electrochemical cells, with two sets of four cellsconnected hydraulically in parallel. For simplicity, only one cell isillustrated. However, the number of cells in the cell pack is determinedby the output volume required from the particular system. Each cell hasan anode chamber 62 and a cathode chamber 64 and the flow of salinesolution is split such that the greater portion is fed to the anodechamber 62 and the lesser portion is fed to the cathode chamber 64. Inthis embodiment, approximately 90% of the saline solution is passedthrough the anode chamber(s) with the remainder passed through thecathode chamber(s). The flow rate of saline solution through the cathodechamber is much lower than for the anode chamber and the pressure in thecathode chamber is also lower.

As the saline solution flows through the electrolytic cells, a fixedcurrent of between 7-9 amps (typically 8 A) is applied to each cellcausing electrolysis of the saline solution thereby generating availablefree chlorine in the resulting anolyte, elsewhere generally referred toas the output solution. In order to produce output solution at arelatively neutral pH, namely between 5 and 7, the pH of the outputsolution is at least partially controlled by dosing a portion of thecatholyte to the inlet stream 58 for the anode chambers 62. Thecatholyte is dosed to the inlet stream 58 by an adjustable peristalticpump 66 and the dosing rate is increased or decreased to achieve thetarget pH. In this way, the system is also adapted to cope with varyingalkalinity of the input potable water. The remaining catholyte which isnot dosed into the input stream 58 for the anode chambers 62 is directedto waste, if necessary diluting it prior to disposal.

Since the flow rate of the saline solution into the cathode chamber 64also has an influence on the pH of the output solution, a flow regulator68 is provided to control the flow of saline entering the chamber. Theflow regulator 68 can be manually adjusted if there is a variation ininput water quality. Output solution is fed from the outlet of the anodechambers 62 of the cell pack into an intermediate weir tank 70.

The pH and redox potential of the output solution in the weir tank 70are measured by a pH meter 72 and a redox probe 74 respectively. If thepH and redox potential do not fall within the desired parameters, avalve 76 is opened and the contents of the weir tank 70 are drained towaste. The contents of the tank 70 are drained to waste in any event ifthey have remained in the tank for about three hours. The pH meter 72 islinked to pump 66 to adjust the level of catholyte dosed to the anodechambers 62 thereby enabling the pH of the output solution to beadjusted to bring the output solution within the desired pH range. Ifthe pH and redox potential of the output solution are determined to fallwithin the desired parameters, confirming that the output solution hasthe necessary biocidal efficacy, the valve 76 is kept closed and theoutput solution is allowed to fill the weir tank 70 until it reaches alevel where it floods over into a storage tank 78. The weir tank 70includes a level detector 80 for monitoring when the level of outputsolution in the tank falls to a predetermined low level. When the lowlevel detector 80 is activated, the production of sterile rinse water isstopped.

Provided the pH meter 72 and the redox probe 74 confirm that the outputsolution has the desired parameters, a corrosion inhibitor, such as amixture of sodium hexametaphosphate and sodium molybdate, is dosed as asolution from a storage container 82 into the output solution in theweir tank 70 by a peristaltic pump 84. A sensor 86 is mounted in thestorage container to monitor low levels of inhibitor and trigger analert mechanism which alerts the system that there is a need forinhibitor to be supplied to the storage container 82.

In specification output solution spills from the weir tank 70 into thestorage tank 78 where it remains until a demand for it is received. Forexample, when it is required for a cycle of a washer-disinfectermachine, the system receives a demand signal from a washing machineinterface control module triggering operation of a dispensing pump 88.Typically, the dispensing pump 88 is rated so that it can supply outputsolution to three washing machine vessels of 25 litre capacity in 180seconds (1500 liters per hour, 3 bar line pressure). The capacity of thestorage tank 78 is therefore such that it too can fulfil the volumerequirement.

The storage tank 78 includes various level detectors for monitoringliquid levels in the tank. A level detector 90 is activated by an extrahigh level of output solution within the tank, raising an alarm andstopping production. A level detector 92 is activated before thedetector 90 as the volume of output solution rises in the storage tank78 and simply stops production. As the output solution is dispensed andafter a period of time below the level of detector 92, production ofoutput solution is recommenced. A low level detector 94 is activatedwhen the level of the output solution falls to a low level, raising analarm and preventing further dispensing to the machine.

A pH probe 96 for monitoring the pH of the output solution is providedwithin the storage tank 78 so that if the pH of the output solutiondrops out of specification, it is routed to waste by a valve 98 locatedon the outlet of the storage tank 78. In addition, if the outputsolution has been stored for 24 hours, it is similarly routed to waste.In this way, output solution which is out of specification is neverdispensed. In order to monitor the flowrate and amount of outputsolution dispensed from the storage tank 78, a flow meter 100 is linkedto ‘no flow’ and leak detection routines within a user/service interfaceto alert the system, for example, that the discharge valve 98 is closedduring a requested discharge, or that an unrequested discharge isoccurring.

Since the output solution held in the weir tank 70 is never more thanthree hours old, it is used to produce bacteria-free rinse water. Freshoutput solution is dosed at a predetermined rate from the weir tank 70to a rinse water storage tank 102 via a peristaltic pump 104. Filteredpotable water flows into the tank 102 through a valve 106 where it mixedwith and dilutes the output solution to a concentration of about 2%. Ifthe local water supply is of poor quality, a higher concentration ofoutput solution in the rinse water, for example a 5% solution, ispreferred. Accordingly, the dosing rate of pump 104 is determined by theincoming potable water supply and is monitored by a flowmeter 108. Bothpotable water and output solution are added to the rinse water storagetank 102 simultaneously and a minimum standing time of two minutes isalways allowed before dispensing the resulting mix. This ensuressufficient contact time for the output solution to diffuse in andactivate the potable water. Rinse water is stored in the rinse waterstorage tank 102 until it is required by, for example, an endoscopewashing machine. A dispensing pump 110 is activated on receipt of ademand signal from a washing machine interface control module. As withthe dispensing pump 88, the dispensing pump 110 is similarly rated tomeet the demand of filling three washing machine vessels of 25 literscapacity in 180 seconds (1500 liters per hour, 3 bar line pressure) andthe capacity of the rinse water storage tank 102 is also dictated bythis typical demand scenario.

The rinse water tank 102 is provided with a plurality of level detectorsto monitor levels of rinse water. A level detector 112 is activated whenthere is an extra high level of rinse water the tank 102, alerting thesystem and stopping any further production of rinse water. Another leveldetector 114 monitors high rinse water level in the tank 102 and whenactivated stops rinse water production. After a predetermined period oftime has elapsed and when the rinse water level has fallen, the highrinse water level detector 114 is deactivated and the production ofrinse water is recommenced. When there is only a low level of rinsewater in the tank 102, a level detector 116 is activated raising analarm and preventing further rinse water from being dispensed.

The flowrate and total rinse water dispensed is monitored by a flowmeter118, which also is used in ‘no flow’ and leak detection routines linkedto the user/service interface (FIG. 1). By automatic monitoring ofliquid levels in the weir tank 70, the storage tank 78 and the rinsewater tank 102, and by discharging the output solution and rinse waterperiodically, the system is able to self-adjust to allow it to meetdemand at all times. Gases generated by the electrolytic reaction in thecell pack, mainly hydrogen and chlorine, are vented through a carbonfilter located above the weir tank 70 and rinse water tank 102 to reducethe quantity of chlorine which escapes.

The system also includes a drip-tray provided with leak detection meansin communication with the user/service interface (FIG. 1). The drip trayis a shallow vessel housing two level detectors 120, 122, one being alow level detector and the other an extra high level detector. The lowlevel indicator 120 is activated by any small leak within the machineand activates an alarm when the liquid level rises above the detector,but does not halt the production process in any way. However, the extrahigh level detector 122 activates an alarm and halts the production anddispensing of output solution. A manual valve 124 is provided at thebase of the drip tray to allow drainage of the tray.

To maintain the system properly, it is necessary to sterilise thestorage tanks and discharge lines on a regular, typically daily, basis.Output solution having the desired biocidal properties as confirmed byits measured parameters and age is flushed through the filters, tanksand pipework to eliminate bacterial growth in these areas. Inparticular, before the cleaning cycle is commenced, the output solutiontank 78 is replenished to a high level as detected by the detector 92ensuring that sufficient output solution is available for the cycle, andthe pH and redox potential of the output water are confirmed as beingwithin specification by the pH probe 72 and the redox probe 74. The pHand redox potential will change during the cleaning process and need notbe monitored once the cleaning process has commenced. On the other hand,the rinse water tank 102 and the process water tank 14 are drained tolow level prior to commencing the cleaning cycle.

Output solution from the storage tank 76 is routed via a valve 126 tofill the process water tank 14 via a spray bar 128. The spray bar 128causes the output solution to be sprayed onto the tank walls throughoutthe filling process. Once process water tank 14 is full to thepredetermined level, the output solution is pumped by pump 44 throughthe cell pack into the weir tank 70. The output solution is then drainedto waste via the valve 76.

When the “cleaning” output solution reaches a low level in the processwater tank 14 as detected by the level detector 26, the tank 14 isre-filled with output solution via the valve 126. Output solution isthen pumped by the pump 44 from the process water tank 14 and the valve46 is opened to divert the output solution to the rinse water tank 102via a spray bar 130. When the rinse water tank 102 is filled, the tankis held full for about five minutes in anticipation of a demand to flushthe rinse water line. If no signal is received, the rinse water tank 102is allowed to drain along with the process water tank 14 and the storagetank 76.

FIG. 3 shows a dispenser 200 for uniformly dispersing two miscibleliquids. The dispenser 200 is in the form of an elongate tube 202 havingan open first end 204 and a second end 206 closed by an end cap 208. Thetube 202 is provided with a row of perforations 210 substantially alongits length. In use in the method of the invention, the open first end204 of the dispenser 200 is fixed to the end of the feed line for theconcentrated salt solution which is pulse fed from the make up tank 20(FIG. 2) by the peristaltic pump 48. The dispenser 200 is located in andaligned with the flow path of process water which is continuously pumpedfrom the process water tank 14 by the pump 44. As the pulses ofconcentrated salt solution arrive in the dispenser 200, the solution isforced out through the perforations 210 into the process water flow. Theresulting saline solution is of substantially homogeneous concentrationby virtue of the mixing pattern achieved by the dispenser 200.

The dilution of the saturated salt solution is determined by the lengthof the dispenser 200, or rather the length over which the perforationsare provided, the pulse rate of the saturated salt solution and thevelocity of the process water.

FIG. 4 shows an electrolytic cell 300 as used in the present invention.The cell 300 comprises co-axial cylindrical and rod electrodes 302, 304respectively, separated by a semi-permeable ceramic membrane 306co-axially mounted between the electrodes thus splitting the spacebetween the electrodes to form two chambers 308, 310. The cylindricalelectrode 302 forming the anode is typically made from commercially puretitanium coated with an electrocatalytic (active) coating suitable forthe evolution of chlorine from a chloride solution. The rod electrode304 forming the cathode is made from titanium and machined from an 8 mmstock bar to a uniform cross-section over its effective length, which istypically about 210 mm.+−0.5 mm. The semi-permeable ceramic membrane 306forming a separator and creating the anode and cathode chambers 308 and310 is composed of aluminium oxide (80%), zirconium oxide (18.5%) andyttrium oxide (1.5%), and has a porosity of about 50-70%, a pore size of0.3 to 0.5 microns and a wall thickness of 0.5 mm+0.3 mm/−0.1 mm. Theceramic of the membrane 306 is more fully disclosed in the specificationof patent application number GB 9914396.8, the subject matter of whichis incorporated herein by reference.

The cell 300 is provided with entry passages 312, 314 to permit thesaline solution to enter the cell 300 and flow upwards through the anodeand cathode chambers 308 and 310 and is discharged as anolyte andcatholyte through exit passages 316, 318 respectively. The anolytecontaining available free chlorine constitutes the output solution.

As previously described, in order to provide a useful amount of outputsolution within a reasonable period of time, a group of cells areconnected together to form a cell pack. For example, a cell packcomprising eight cells connected together in parallel hydraulically andin series electrically is capable of generating about 200 liters/hour ofoutput solution.

Although the invention has been particularly described, it should beappreciated that the invention is not limited to the particularembodiments described and illustrated, but includes all modificationsand variations falling within the scope of the invention as defined inthe appended claims. For example, means other than the elongate,perforated dispenser described for mixing the concentrated salt solutionwith process water to produce a homogeneous saline solution may be used.Indeed, the concentrated salt solution can be continuously fed into astream of process water rather than being pulse fed. In addition, whilea weir tank is described as being particularly suitable for providingintermediate holding means for the output solution, other types ofholding means may be used, such as a more conventional tank havingappropriate outlet means for transferring its contents to the outputsolution storage tank. The cell separator can be made of ceramics otherthan the aluminium oxide, zirconium oxide and yttrium oxide ceramicdescribed and of any other suitable semi-permeable or ion-selectivematerial.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A biocidal solution having a pH of from 5 to 7 and an available freechlorine (AFC) concentration of about 3 parts per million (ppm) to 300ppm.
 2. The biocidal solution of claim 1, wherein the AFC concentrationis approximately 100 to 250 ppm.
 3. A method of disinfecting a medicalinstrument comprising exposing the medical instrument to the biocidalsolution of claim
 2. 4. A method of treating a venous leg ulcer in apatient comprising administering the biocidal solution of claim 1 to theleg ulcer.
 5. A method of disinfecting poultry or fish comprisingexposing the poultry or fish to the biocidal solution of claim
 1. 6. Amethod of breaking down bacterial biofilm comprising exposing thebiofilm to the biocidal solution of claim
 1. 7. A method of producingthe biocidal solution of claim 1 comprising: providing anelectrochemical cell comprising an anode chamber having an anode and acathode chamber having a cathode; supplying a saline solution to theanode chamber and the cathode chamber; applying a current across thecell between the anode and the cathode; and obtaining the biocidalsolution from the electrochemical cell.
 8. The method of claim 6,wherein obtaining the biocidal solution comprises obtaining the solutionfrom the anode chamber.
 9. The method of claim 6, wherein applying acurrent comprises applying a current of 8 amps when the surface area ofthe anode is approximately 100 cm².
 10. The method of claim 6, wherein alarger proportion of the saline solution is fed to the anode chamberthan is fed to the cathode chamber.
 11. The method of claim 9, wherein80% of the saline solution is fed to the anode chamber and 20% of thesaline solution is fed to the cathode chamber.
 12. The method of claim6, further comprising applying a substantially constant current acrossthe cell between the cathode and the anode and passing a substantiallyconstant throughput of chloride ions through the electrochemical cell.13. The method of claim 6, wherein supplying a saline solution comprisesforming a saline solution by combining a diluent potable tap water and aconcentrated salt solution.
 14. The method of claim 12, wherein forminga saline solution comprises feeding the concentrated salt solution bypulsatile means into a flow of the diluent water to produce the salinesolution.
 15. The method of claim 13, wherein the concentrated saltsolution is pulsed into a continuous flow of the diluent water through aplurality of apertures along the flow path to produce a uniformly mixedsaline solution.
 16. The method of claim 6, wherein supplying a salinesolution comprises supplying a substantially constant concentration of asaline solution to the anode chamber and the cathode chamber.
 17. Themethod of claim 6, further comprising recirculating a portion of thecatholyte produced in the cathode chamber into the anode chamber.